High efficiency lighting sources are continually being developed to compete with traditional area lighting sources, such as fluorescent lighting. For example, while light emitting diodes have traditionally been implemented as indicator lighting and numerical displays, advances in light emitting diode technology have fueled interest in using such technology in area lighting. Light Emitting Diodes (LEDs) and Organic Light Emitting Diodes (OLEDs) are solid-state semiconductor devices that convert electrical energy into light. While LEDs implement inorganic semiconductor layers to convert electrical energy into light, OLEDs implement organic semiconductor layers to convert electrical energy into light. Generally, OLEDs are fabricated by disposing multiple layers of organic thin films between two conductors or electrodes. When electrical current is applied to the electrodes, light is emitted from the organic layers. Unlike traditional LEDs, OLEDs can be processed using low cost, large area thin film deposition processes. OLED technology lends itself to the creation of ultra-thin lighting displays that can operate at lower voltages than LEDs. Significant developments have been made in providing general area lighting implementing OLEDs.
However, while traditional OLEDs having a relatively low efficacy (e.g. 3-4 lumens per watt) may be able to achieve sufficient brightness for area lighting at low voltages, the operating life of the OLED may be limited due to the heat generated by the high power level and relatively low efficiency of the device. To provide commercially viable light sources implementing OLEDs, the efficacy of the devices may be improved to reduce the heat generation when operating at a brightness sufficient to provide general illumination. Further, to improve the efficiency of the OLED as a general lighting source, light loss mechanisms may be minimized to increase the amount of useful, ambient light that is converted from the electricity.
Conventional OLEDs generally emit approximately 17-33% of the light generated within the organic layers of the OLED. The reduction in light generated within the OLED to the light emitted to the ambient environment is generally caused by loss mechanisms within the OLED. There are a number of light loss mechanisms in OLEDs, as will be discussed further, herein. The production of high efficiency lighting sources for general illumination suggests that light loss mechanisms within the light source be minimized. In flat panel electroluminescent devices (i.e., devices which convert energy into light), light is generated within a dielectric medium. A significant fraction (greater than 40%, for example) of the light that is generated may not be coupled into the ambient, but rather may be lost due to internal reflection. In addition, depending upon the thickness of the substrate on which the organic semiconductor and electrodes are disposed, a significant fraction of the light (greater than 10%, for example) may emerge from the sides of the device, which may be less useful for general illumination purposes. In large volume lighting applications such losses may represent a significant amount of wasted energy consumption.
To reduce the amount of light lost in OLEDs, a number of different techniques have been implemented in conventional OLED devices. For example, scattering particles may be implemented with devices having vertically reflective sides. Further, mirrors angled at 45 degrees may be implemented to capture light emitted from the pixels on the substrate, as can be appreciated by those skilled in the art. Still further, wedged and ridged-wedge light guides may be implemented. In these implementations light may be deliberately injected from one side of the OLED to provide even, uniform illumination. However, these techniques are generally used to provide backlighting for small areas
Further, conventional OLED devices which have been implemented for display applications generally address the preservation of the underlying OLED spatial construction. That is to say that a user of an OLED display may typically be interested in not only seeing a “pixel” but also knowing its exact spatial delineation. Conversely, in a lighting applications, diffuse area lighting that preserves little of the spatial information contained in the OLED patterning may not be important, and in some applications a diffuse, structure-less source may be preferred.